In a numerical control machining apparatus controlled by an NC (hereinafter referred to as "an NC machining apparatus", when applicable) numerical data instruct the position of a tool with respect to the workpiece so that the workpiece is machined under control of numerical values which are calculated by the NC. Accordingly, with the NC machining apparatus, a workpiece can be machined into an intricate product with ease and with high accuracy, and the productivity can be increased. In general, an NC machining apparatus, as shown in FIG. 1, comprises: an NC 20 for performing calculation on numeral data commands inputted through a terminal 10; and a machining device 30 which is controlled by the output data of the NC 20. The NC 20 comprises: an input unit 21 to which commands are inputted; an arithmetic unit 22 for performing operations on commands from the input section 21; a memory unit 23 for storing operation results outputted by the arithmetic unit 22 and commands from the input unit 21; a control unit 24 for controlling the operation of the arithmetic unit; and an output unit 25 for delivering operation values such as operation results provided by the arithmetic unit 22 out of the NC 20. On the other hand, in the machining apparatus 30, a tool 31 is held by a tool holder 32 which is secured to the chuck of a spindle 33, and the latter 33 is rotated by a spindle motor 34 which is driven by signals from the output unit 25 in the NC 20. A workpiece 40 is fixedly set on the table 35 of the apparatus 30 with a jig or the like. In FIG. 1, reference numeral 36 designates a ball screw for moving the table 35 along the X-axis. The ball screw 36 is driven through a gear box 37 by an X-axis feed motor 38, which is driven by signals from the output unit 25. Mechanisms (not shown) for moving the table 35 along the Y-axis and the Z-axis are provided, which are similar to the above-described mechanism for moving the table 35 along the X-axis. These mechanisms are also controlled by signals from the NC 20.
In the above-described NC machining system using the NC machining apparatus, machining time can be reduced by heavy load machining; however, the system suffers from a drawback that, when a load applied to the tool exceeds its allowable value, the tool is broken. This drawback may be eliminated by decreasing the load; however, in this case, the machining speed becomes low.
According, in the conventional NC machining system, in order to increase the machining efficiency and to prevent the tool from being damaged, machining conditions calculated according to the tool's allowable load are stored in the memory unit 23 of the NC 20, so that machining is carried out according to the machining conditions thus stored.
Thus, the conventional NC machining system is disadvantageous in the following points: When the load of the tool exceeds its allowable value abruptly, the tool may be broken, because the change of the load of the tool cannot be detected during machining. If, in order to prevent the trouble that, when the load of the tool changes abruptly, the tool is broken, the machining conditions are determined with the load of the tool being much lower than the allowable load, then the machining efficiency is lowered.